In vivo studies of the sis transforming gene, either from the simian sarcoma virus (v-sis) or from human cells (c-sis), continue to yield data on the unique biologic effects of this important oncogene. When introduced to newborn mice in an infectious viral form, it produces sarcomas after a very long incubation period in either euthymic or athymic animals. Cells grown from the primary sarcomas are transplantable to athymic mice only, where they produce slow growing local sarcomas as well as spleen metastases. Studies on malignancy of transformed cultures in vivo, using human cells transformed by oncogenic viruses or rodent cells transfected with human oncogenes are producing valuable data on the nature of neoplastic progression.